The present invention relates to a hydraulic control system for an automatic transmission, and more particularly relates to such an automatic transmission hydraulic control system which is capable of providing effective and fail safe anti creep control and also hill holding control.
The present invention has been described in Japanese patent applications Ser. Nos. Showa 61-250423 (1986), Showa 61-250424 (1986), Showa 61-250425 (1986), and Showa 61-250426 (1986), all of which were filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the texts of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
When a vehicle equipped with an automatic transmission is stationary or nearly stationary, as for example when waiting at traffic lights, it is per se conventional to provide some form of anti creep action, in order to prevent the vehicle from springing forward on the occasion of some inadvertence of the driver. In other words, even though a manual range setting means controlled by the driver of the vehicle is still set to a forward vehicle operational range such as "D" or drive range, it is per se known to set the transmission of the vehicle, effectively, to a neutral operational speed stage or to an operational speed stage which is equivalent to neutral. This can be done, for example, by releasing a forward clutch of the transmission, or by much reducing the torque transmission capacity of such a forward clutch so that it slips to a certain degree, or by reducing the torque transmission capacity of a hydraulic coupling of the transmission so that the engine torque is at least not completely transmitted to the gear transmission mechanism of the transmission.
Further, when such a vehicle equipped with an automatic transmission is stationary or nearly stationary upon an upward slope, as for example on a hill, in order to prevent the vehicle slipping backwards and in order to facilitate the process of starting up again to progress up the slope, it is per se conventional to provide a so called hill hold control mode, in which, for example, one of the shift brakes incorporated in the gear transmission mechanism of the transmission is engaged, so that the output shaft of the transmission is not able to rotate in the direction to allow the vehicle to move in the backwards direction. Such a type of construction is disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 61-55455 (1986), which it is not hereby intended to admit as prior art to the present patent application except to the extent in any case mandated by applicable law.
However, various problems arise with such a type of construction.
Namely, this type of hill hold control is really only applicable when the vehicle is stationary or substantially stationary and anti creep control is being carried out. If this is not the case, then the engagement of the shift brake for providing such hill hold control will amount to the engagement of a previously prohibited combination of friction engaging devices of the gear transmission mechanism, so that when said shift brake is engaged said gear transmission mechanism will be subjected to a massive torque shock, and this can seriously affect the durability of said gear transmission mechanism and of the friction engaging devices incorporated in it.
For example, to consider a particular typical constructional case for such a transmission, suppose that the shift brake for providing such hill hold control is a second coast brake which prevents the rotation of a sun gear of the gear transmission mechanism, then if, for example, while the transmission is operating in its "D" or drive range and the gear transmission mechanism is engaged to its third speed stage and the vehicle is in a decelerating operational condition, mistakenly said second coast brake is engaged, a direct clutch which is provided for directly coupling said sun gear with the input element of that portion of the gear transmission mechanism which provides the third speed stage and also said second coast brake will be applied simultaneously, and this will effectively amount to an attempt to prevent the power output shaft of the transmission from rotating. Accordingly, a very large torque shock will be engendered in the transmission, and this will be transmitted to the various members of the power train such as in particular the friction engaging devices of the gear transmission mechanism, thus adversely affecting the durability of the transmission as a whole.
Further, a subsidiary problem can occur, that if the hill hold control is undesirably released while the engagement pressure of such a forward clutch of the transmission is still being kept relatively low in order to provide anti creep control, there is a risk of the vehicle slipping in the reverse direction.
Yet another problem that can occur with such a system is that relatively sudden changes in the torque transmission capacity of such a forward clutch which is being used for providing anti creep control can themselves cause relatively high transmission torque shock.
A further, and final, problem that gives difficulties is that, for the above described type of hill hold control, the engagement hydraulic fluid pressure for a shift brake such as a second coast brake is typically determined as being the line pressure of the transmission; but, in fact, when for example the transmission is set to its second speed stage while operating in its "S" or second range, it would actually be preferable to determine said engagement hydraulic fluid pressure for said second coast brake to be a pressure such as a second coast modulate pressure, which is different from the line pressure and substantially lower than said line pressure.